Multielement radio-frequency antenna structure having helically coiled conductive elements

ABSTRACT

A multielement antenna structure is provided which may be fabricated with a predetermined characteristic impedance for impedance matching purposes. The several elements comprise elongated electrical conductors formed in a helix and encased in a supporting body formed from a hardenable resin matrix. A broadband frequency characteristic is obtained through appropriate selection of conductor diameters that will result in resonance of the several conductors at different respective frequencies within the design frequency band.

United States Patent [72] Inventors Richard J. Franck;

Clara A. Franck, both of 11855 Broad St., Pataskala, Ohio 43062 211 App]. No. 670,142 [22] Filed Sept. 25, 1967 [45] Patented July 27, 1971 [54] MULTIELEMENT RADIO-FREQUENCY ANTENNA STRUCTURE HAVING HELICALLY COILED CONDUCT IVE ELEMENTS 9 Claims, 5 Drawing Figs.

[52] US. Cl 343/873, 343/843, 343/895 [51] Int. Cl H01q 1/38, HOlq l/40, H01q 9/32 [50] Field of Search 343/715, 895, 873, 872, 843

[56] References Cited UNITED STATES PATENTS 3,230,540 1/1966 Endo et a1 343/895 X 3,102,268 8/1963 Foley 343/895 3,083,364 3/1963 Scheldorf... 343/895 X 2,611,868 9/1952 Marston 343/895 X 1,284,155 11/1918 Shartzer 343/895 X 2,938,210 5/1960 Harris 343/895 2,210,066 8/1940 Cork et a1. 343/905 X 2,681,412 6/1954 Webster 343/899 X 2,682,608 6/1954 Johnson 343/803 X 2,755,469 7/1956 Etheridge 343/895 FOREIGN PATENTS 34,068 3/1929 France 343/895 OTHER REFERENCES Li et al., Helical Folded DiPoles & Unipoles, Pro. National Electronics Conf. 1957, pp. 89-105 Roberts, W. Van Impedance Transformation In Of A Pro. Ire, 9-1950 pp. 1042- 1047 Roberts, W. Van 8,, Input Impedance Of A Folded DiPoIe," RCA Review 6-1947, pp. 289- 300 Richard, L. R., Parallel DiPoles Of 300-Ohm Ribbon," American QST 3-1957 p. 14.

King, R. W. P. Linear Antennas," Harvard U. Press 1956, pp. 273- 277 Weeks, W. L., Electromagnetic Theory For Engineering Applications," John Wiley & Sons 1964, pp. 163- 167 Kraus, .l. D., Antennas," McGraw Hill, l950, p. 179

Tice & Kraus: The Influence Of Conductor Size On The Properties Of Helical Beam Antennas Proceedings Of The Institute Of Radio Engineers, November 1949, page 1296 Primary Examiner-Eli Lieberman Assistant Examiner-Wm. H. Punter Attorney-Mahoney, Miller & Stebens ABSTRACT: A multielement antenna structure is provided which may be fabricated with a predetermined characteristic impedance for impedance matching purposes. The several elements comprise elongated electrical conductors formed in a helix and encased in a supporting body formed from a hardenable resin matrix. A broadband frequency characteristic is obtained through appropriate selection of conductor diameters that will result in resonance of the several conductors at difierent respective frequencies within the design frequency band.

PATENTEflJuLemn 3 273 sum 1 0F 2 INVENTORS RICHARD J. FRANCIS 8 CLARA A. FRANCIS BY BYMAHONEY, MILLER a RAMBO 9 ATTORNEYS PATENTED JUL2 7 I971 SHEET 2 (IF 2 mmzOlmwm FREQUENCY INVENTORS RICHARD J. FRANCIS 8 CLARA A. FRANCIS FREQUENCY mmzOmmmt MULTELEMENT RADIO-FREQUENCY ANTENNA STRUCTURE HAVING I'IELICALLY COILED CONDUCTIVE ELEMENTS DETAILED DESCRIPTION The antenna structure of this invention is primarily adapted to mobile transmitter-receiver installations such as would be utilized for citizens-band operations in connection with automotive vehicles although the antenna structure is adaptable to other frequency-band allocations. In the usual installations of this type, the antenna is a single element, electrically conductive element effective as both a receiver and radiator of electromagnetic wave energy and is of a construction to accommodate the forces that may be applied as a consequence of vehicular movement.

The most common mobile antennas are an electrical quarter wave in length and metallic. They range in length from about 9 feet for 27 megacycles to about 6 inches for 470 megacycles. They are vertically mounted and supported only at the bottom. They are end-fed.

Vertical quarter wave antennas are unwieldy when more than 9 feet long. However, an electrical quarter wave antenna in the l to 500 megacycle range may be physically shortened by adding inductance in series. Conversely, the physical length, commonly called aperture, is increased by adding capacitance in series.

Quarter wave antennas are desirable because when end-fed they approach resonance. Resonance is a state where inductance and capacitance reactances are equal and as a result the total impedance is its direct current resistance.

In two-way radio communications the transceiver and antenna are connected with coaxial cable, and the most commonly used coaxial cable has a characteristic impedance of 52 ohms. The output stage of the transceiver is adjustable to 52 ohms. However, the terminal impedance of an end-fed quarter wave is well below 52 ohms, perhaps as low as ohms.

Maximum power transmission results when the terminal impedance of the end-fed quarter wave antenna matches the impedance of the coaxial transmission line. With the single element antennas of prior art the impedance mismatch is great enough to seriously impede the efficiency of power transferral. Some installations are operated inefficiently with this mismatch, while other installations rely on complex impedance matching networks to correct this mismatch.

The antenna structures of this invention may be readily adapted for use throughout the frequency spectrum of l to 500 megacycles.

It is, therefore, an important object of this invention to provide a novel antenna structure which may be constructed to provide a more advantageous impedance match with that of a connecting coaxial transmission cable.

It is another important object of this invention to provide a novel antenna structure for mobile, vehicular installations that comprises a multiplicity of electrically conductive elements encased in a structurally supporting body of synthetic resin capable of withstanding the structural load forces that may be imposed in a mobile, vehicular installation.

It is another object of this invention to provide an antenna structure having a multiplicity of helically coiled, electrically conductive elements and having a desired characteristic impedance for a particular design frequency band.

It is a further object of this invention to provide an antenna structure for a particular design frequency band and having a multiplicity of electrically conductive elements of different diameters to provide a relatively wide band width while main taining a relatively high response and radiation characteristic for the entire frequency band.

It is also an object of this invention to provide an antenna structure for a particular design frequency band and having a multiplicity of electrically conductive elements of different lengths to provide a wide band width while maintaining a relative high response and radiation characteristic for the entire band.

' structure embodying this invention.

FIG. 2 is a transverse sectional view taken along line 2-2 of FIG. 1.

FIG. 3 is a diagrammatic illustration of the electrical equivalent circuit of the antenna structure of FIG. 1.

FIGS. 4 and 5 are graphic representations of the frequency response characteristic of electrically conductive elements to electromagnetic wave energy.

Having reference to FIGS. 1 and 2 of the drawings, an antenna structure embodying this invention is illustrated in detail. This antenna structure comprises a multiplicity of electrically conductive elements, indicated generally at 10, encased in a structurally supporting body which may be conveniently described as having a central core 11 and an outer, coaxial sheath 12 although the core and sheath may be integrally formed in a completed antenna structure. In this embodiment, there are two electrically conductive elements, designated by the numerals 13 and 14, which are effective, at the design radio frequencies, for radiation or reception of electromagnetic wave energy. Each element, 13 and 14, is formed from a relatively small diameter electrical conductor, such as a copper wire, with the two conductor elements being helically coiled to define an elongated cylinder having a cylinder diameter usually greater than the conductor diameter.

These electrically conductive elements 10 are conveniently wound on the preformed central core 11 in the manufacture of the antenna structure with the core being formed from a material having the necessary structural strength when thus formed to support the coiled conductive elements 13 and 14. In accordance with this invention, the conductive elements 13 and 14 are formed from small diameter copper wire, such as No. 24, and consequently are not self supporting. Furthermore, the length of the completed antenna structure approximates one-fourth of the wavelength of the electromagnetic wave energy at which the antenna is designed to operate in the usual construction and, since this length is usually of the order of 38 feet in the citizens-band frequency spectrum of the order of 27 megacycles, this precludes reliance on the structural strength of the conductive elements for structural integrity of the antenna structure. Construction of the structurally supporting body will be subsequently described in further detail.

The two electrically conductive elements 13 and 14 are wound in the same plane in side-by-side parallel relationship but each turn or coil of a pair of parallel elements is longitudinally spaced from an adjacent coil by a predetermined distance. This longitudinal Spacing is of the order of one-sixteenth inches in one embodiment of the antenna structure with the coil diameter being of the order of one-eighth inch. At least one of the conductive elements, 13 or 14, may be provided with a dielectric sheath 16 which assures spacing of one element relative to the other and thus effectively insulates the two adjacent elements against transmission of electrical current therebetween. This dielectric sheath 16 in the present embodiment comprises a suitable varnish; however, other well-known materials which do not provide electromagnetic shielding may also be utilized. If desired, the dielectric sheath 16 may be omitted if the element spacing is otherwise maintained or both elements may be provided with a dielectric sheath.

When elements 13 and 14 are of dissimiliar cross sections and are helically wound over a core of constant diameter, each turn of the larger cross section is longer than a turn of the smaller diameter.

In completing the electrical circuit connections of this antenna structure, both conductive elements l3, 14 are electrically interconnected at one end as is mechanically illustrated in FIG. 1 and electrically illustrated in FIG. 3. This may be mechanically accomplished as shown in FIG. 1 by soldering portions of two exposed conductive elements together and to a mounting ferrule 15 and forming the feed point of the antenna. The opposite ends of the conductors are not electrically connected and the electrical circuit thus presented, as shown in FIG. 3, consists of a pair of parallel, helically coiled, electrical conductors which are open-circuited at one end and will prevent current circulation within the two conductors.

Through selection of conductive elements 13 and 14 of ap- In the usual mobile installation, the desired characteristic" impedance for proper matching is 520 as this is the impedance of the most commonly used commercially available coaxial transmission cable. A coaxial cable C of this type is shown in FIG. 3 as being electrically connected to the antenna structure.

As in the case of conventional antennas, the antenna structure of this invention will preferably be a quarter wave length for the specific design frequency band.

One of the parameters controlling the physical length of an end-fed electrical quarter wave antenna is the diameter of the conductor. For a given electrical quarter wave the physical length of the conductor decreases as the conductor diameter increases, but not as a straight line function. FIG. 4 illustrates this condition. Curve M shows the response of a conductor of a given diameter, while curve N is the response of a conductor of another diameter. FIG. 4 shows that their resonant frcquencies are at different frequency values in the spectrum, and illustrates how a multiplicity of conductors of dissimilar diameters broadens the effective band width of an antenna.

FIG. 5 illustrates how conductors of different physical lengths have their maximum response at dissimilar frequencies when end-fed as quarter wave antennas. Curves P and 8 represent conductors of different physical lengths, and their resonant frequencies may be widely displaced.

The supporting body for the conductive elements 13 and 14 and which comprises a central core 11 and an outer coaxial sheath 12, in accordance with this invention, is preferably formed from a solidified matrix of a hardenable synthetic resin. The resin selected must have suitable characteristics as to mechanical and electrical properties. Mechanical characteristics of flexural strength and modulus must be sufficient to withstand static and dynamic loads that may be imposed in vehicular installations. Electrical properties must include adequate dielectric strength and electrical transparency at radio frequencies. Preferably, the resin matrix which may comprise a thermosetting polyester or epoxy, also includes strands of fiber glass 17 distributed throughout the body to enhance the mechanical properties of the antenna structure. These strands may be oriented longitudinally and some strands may be oriented helically in the surface.

A metallic mounting ferrule is provided to facilitate attachment of the end-fed quarter wave antenna to the vehicle. This ferrule 15 is formed with a central socket in which an end of the antenna structure is inserted and secured as by a suitable adhesive or bonding material. The ends of the conductive elements 13 and 14 which are exposed extend through an aperture 18 andmay be soldered to the ferrule 15 as at 19.

The central core 11 may also be comprised of a ferrite material to further enhance the electrical properties of the antenna structure. Ferrites such as the iron carbonyls and magnetic iron oxide may be in particulateform embedded in the resin matrix.

Although the illustrated embodiment comprises only two conductive elements, 13 and 14, the number of elements may be increased. For example, three such elements may be formed into a similar helical coil to obtain the desired characteristic impedance with the desired frequency band response.

It will be readily apparent that a novel antenna structure is provided which may be readily constructed with the desired terminal impedance. Utilizing conductive elements of different diameters and of different lengths provides a broad band frequency response.

According to the provisions of the patent statutes, the principles of this invention have been explained and have been illustrated and described in what is now considered to represent the best embodiment. However, it is to be understood that, within the scope of the appended claims, the invention ma be practiced otherwise than as specifically illustrated and described.

We claim: Having thus described this invention, what we claim is:

1. A radio-frequency antenna structure comprising at least two elongated, electrically conductive elements disposed in the same plane in side-by-side relationship and formed into respective cylindrical helixes of the same pitch and internal diameter, each of said elements being electrically insulated from the other throughout the length thereof and being electrically connected together at only one end thereof, which end forms a feed point, said elements being of selected cross-sectional area and relatively spaced to provide a desired characteristic impedance, and a supporting body structure for said electrically conductive elements formed from a dielectric having a relatively low loss characteristic as to electromagnetic wave energy and physical strength characteristics to maintain the physical configuration of the conductive elements and assure structural integrity of the antenna structure.

2. An rf antenna structure according to claim 1 wherein helical spacing of said conductive element groups is of the order of one sixteenth inch.

3. An r--f antenna structure according to claim 1 wherein said electrically conductive elements are of dissimilar crosssectional area to provide a relatively greater effective band width of operating frequencies.

4. An rf antenna structure according to claim 1 wherein said electrically conductive elements are of dissimilar lengths to provide a relatively greater effective band width of operating frequencies.

5. An rf antenna structure according to claim 1 having a core formed from a material with a magnetic permeability greater than unity.

6. An rf antenna structure according to claim 1 wherein said supporting structure is a fiber glass reinforced, synthetic resin matrix having a central core around which said electrically conductive elements are helically wound and an outer sheath in which said core and conductive elements are .encased, said structure including a multiplicity of longitudinally extending strands of fiberglass.

7. An rf antenna structure according to claim 6 wherein said synthetic resin is a thermosetting polyester.

8. An r--f antenna structure according to claim 6 wherein said synthetic resin is an epoxy.

9. An rf antenna structure according to claim 6 in which said fiber glass strands are distributed through the core and sheath of said resin matrix. 

1. A radio-frequency antenna structure comprising at least two elongated, electrically conductive elements disposed in the same plane in side-by-side relationship and formed into respective cylindrical helixes of the same pitch and internal diameter, each of said elements being electrically insulated from tHe other throughout the length thereof and being electrically connected together at only one end thereof, which end forms a feed point, said elements being of selected cross-sectional area and relatively spaced to provide a desired characteristic impedance, and a supporting body structure for said electrically conductive elements formed from a dielectric having a relatively low loss characteristic as to electromagnetic wave energy and physical strength characteristics to maintain the physical configuration of the conductive elements and assure structural integrity of the antenna structure.
 2. An r-f antenna structure according to claim 1 wherein helical spacing of said conductive element groups is of the order of one-sixteenth inch.
 3. An r-f antenna structure according to claim 1 wherein said electrically conductive elements are of dissimilar cross-sectional area to provide a relatively greater effective band width of operating frequencies.
 4. An r-f antenna structure according to claim 1 wherein said electrically conductive elements are of dissimilar lengths to provide a relatively greater effective band width of operating frequencies.
 5. An r-f antenna structure according to claim 1 having a core formed from a material with a magnetic permeability greater than unity.
 6. An r-f antenna structure according to claim 1 wherein said supporting structure is a fiber glass reinforced, synthetic resin matrix having a central core around which said electrically conductive elements are helically wound and an outer sheath in which said core and conductive elements are encased, said structure including a multiplicity of longitudinally extending strands of fiberglass.
 7. An r-f antenna structure according to claim 6 wherein said synthetic resin is a thermosetting polyester.
 8. An r-f antenna structure according to claim 6 wherein said synthetic resin is an epoxy.
 9. An r-f antenna structure according to claim 6 in which said fiber glass strands are distributed through the core and sheath of said resin matrix. 